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The study found only three of the 32 products tested contained the quantity of omega-3 fatty acids claimed on the label. On average, the products contained 68 per cent of what the label claimed.

The study, done by researchers at the university’s Liggins Institute and School of Biological Sciences and the University of Newcastle, also found the majority of the fish oils were considerably oxidised.

The study did not name the brands tested.

A report on the study, published in international science journal Nature, noted how 17 products listed the same concentration of omega-3 fatty acids on the label.

“Such striking uniformity suggests that the companies selling these products have relied on data provided by extractors who supply fish oil to multiple brands,” the report said.

The study found best-before date, cost, country of origin and exclusivity could not be used to determine the quality of fish oil supplements on the market. Those products manufactured in Australia and New Zealand were as oxidised as those from other countries.

Consumer’s CEO Sue Chetwin said the claims being made by manufacturers and distributors needed to be more closely monitored.

“The issue is not with regulation but enforcement,” she said.

The Ministry for Primary Industries (MPI) and Medsafe are following up with the Liggins Institute to get more information on data used in the study.

MPI said in a statement that it was working closely with Medsafe on the requirements of the Dietary Supplements Regulations 1985 and how they apply to these products.